The continuing increase in sophistication of radars and communication systems has created the need for very low noise microwave frequency synthesizers for local oscillator and exciter applications.
The present state-of-the-art in low noise frequency synthesis is dominated by direct analog synthesizers. This architecture is based on frequency addition, subtraction, multiplication and division. Components used in direct analog synthesizers are low-noise oscillators, high stability accurate reference oscillators, amplifiers, frequency multipliers, frequency dividers, mixers, filters and switches. In a mathematical description of direct analog synthesis, mixing is represented by frequency addition or subtraction while generation of harmonic and subharmonic frequencies is represented by multiplication and division, respectively. Closed loop techniques (such as phase-lock or frequency-lock loops) are used in direct analog synthesizers to perform the ancillary functions of frequency synchronization and/or frequency stabilization.
One of the disadvantages of direct analog synthesis is that the low-noise oscillators such as surface acoustic wave (SAW) oscillators.sup.(1), surface transverse wave (STW) oscillators.sup.(2) and crystal oscillators operate at lower frequencies. Specifically, the highest output frequency of low noise crystal oscillators is approximately 100 MHz and the highest output frequency of low-noise SAW and STW oscillators is approximately 1000 MHz. Thus, either frequency multiplication, frequency upconversion or a combination of both are needed for direct analog synthesis of microwave frequencies. In frequency multiplication the phase noise of a low-noise oscillator is degraded by 20.multidot.log(N)dB where N is the frequency multiplication factor. In frequency upconversion, the output frequencies of a lower frequency direct analog synthesizer are upconverted to microwave frequencies by mixing the frequencies of the direct analog synthesizer with the output frequency of a low-noise microwave oscillator or the output frequency of a low-noise lower frequency oscillator followed by a frequency multiplier. Two examples are presented to illustrate the phase noise performance of prior art microwave direct analog synthesizers.
A prior art X-Band direct analog synthesizer 10 is shown in FIG. 1. It consists of a set of four SAW oscillators F1-F4 that are selected either one or two or three at a time by the switch matrix 12 to synthesize 64 equally spaced output frequencies. The frequencies F1, F2, F3, F4 of the SAW oscillators are equally spaced, the spacing being equal to the output frequency spacing F.sub.S of the synthesizer. This relationship is expressed as EQU F4-F3=F3-F2=F2-F1=F.sub.S (1)
All the oscillators must be energized if short frequency switching and settling times are required. The output signal of one selected oscillator is amplified by amplifier 14 and connected to the input of a times-four frequency multiplier 16 the output of which is connected to the first mixer 18. The second signal to this mixer 18 is the direct output of another (or the same) selected oscillator after amplification in amplifier 15. The output signal from mixer 18 is filtered by band-pass filter 20 to yield the sum of these two frequencies. This sum frequency signal is applied to the input of amplifier 22 followed by the second mixer 24 where its frequency is added to the 16th harmonic of the frequency of another (or the same) oscillator selected by switch matrix 12 and amplified in amplifier 26, and multiplied times sixteen by frequency multiplier 28, and amplified in amplifier 30. The sum frequency signal at the output of mixer 24 is filtered by the second band-pass filter 32 and amplified by amplifier 34. This is the output signal F.sub.o of the synthesizer 10.
As shown in the diagram of FIG. 1, all the SAW oscillators F1-F4 are phase-locked by separate narrow band phase-locked loops (PLLs), with a loop bandwidth of approximately 100 Hz, to frequency F.sub.S derived from frequency F.sub.R of the reference oscillator 36 divided by an integer K. The purpose of the PLLs is to accurately control the absolute frequency of each SAW oscillator without affecting its phase noise to ensure that all the oscillators operate at frequencies that are integer multiples of frequency F.sub.S, which is equal to the output frequency spacing of the synthesizer. Phase locking each oscillator to a common frequency is necessary in order to minimize generation of spurious frequencies in the synthesizer and to control the absolute accuracy of each output frequency of the synthesizer. The frequency of each SAW oscillator must therefore be selected to be an integer multiple of frequency F.sub.S. The signal at frequency F.sub.S is fed to the PLL blocks associated with each of the SAW oscillators. Each PLL includes a phase detector which has the frequency F.sub.S as one of its input signals. The other input signal to each of the PLL phase detectors is derived by dividing the frequency of the corresponding SAW oscillator by an integer chosen to make the output frequency of the divider equal to F.sub.S. The phase detector output signal of each PLL is fed to the frequency tuning port of the corresponding SAW oscillator through appropriate loop amplification and filtering. To ensure that the PLLs do not lose lock as a result of SAW oscillator frequency drift over temperature, the temperature of each SAW oscillator is controlled to reduce its frequency drift to a value that is less than its frequency tuning range. SAW oscillator frequency also drifts over time due to aging of components but this drift is typically much smaller than the drift over temperature and will not be considered in this discussion.
If the frequencies of the four SAW oscillators are F.sub.i (i=1,2,3,4) then the output frequency F.sub.o of this synthesizer is given by: EQU F.sub.o =Fi+4Fj+16Fk i,j,k=1,2,3,4 (2)
Assuming that the phase noise levels of all the SAW oscillators are identical, the highest phase noise at the output of the synthesizer occurs when i=j=k. Under this condition EQU F.sub.o =Fi+4Fi+16Fi=21Fi (3)
meaning that the output frequency is the 21st harmonic of the selected SAW oscillator frequency and that the cumulative phase noise at the output of the synthesizer is the phase noise of the SAW oscillator increased by 20.multidot.log(21) dB, which is equal to 26.4 dB. Other synthesizer components may degrade this noise further.
Consider, for example, a synthesizer with 64 X-band frequencies spaced by 10 MHz, from 9870 MHz to 10500 MHz. This would require SAW oscillator frequencies of 470, 480, 490 and 500 MHz. Since the phase noise of a very low noise 500 MHz SAW oscillator is -167 dBc/Hz at 10 kHz offset from the carrier, an increase of 26.4 dB yields phase noise of -140.6 dBc/Hz. Because of all other noise contributors (amplifiers, mixers, switches, etc.) the predicted phase noise at the output of this microwave synthesizer is typically -139.6 dBc/Hz at 10 kHz offset from the carrier.
Another prior art direct analog synthesizer topology is shown in FIG. 2 wherein like components to those in FIG. 1 will not be further described (for simplification) and are designated by the same reference numeral with a zero suffix. In this topology the L-band output frequency of a direct analog synthesizer 100 (similar to the one in FIG. 1), capable of generating 16 frequencies (e.g. 1450 MHz to 1600 MHz, spaced by 10 MHz), is mixed with a microwave frequency F.sub.M (e.g. 8.5 GHz) of a low-noise microwave oscillator 400, in this example a dielectric resonator oscillator (DRO). The key components of the DRO are a low phase noise microwave amplifier, a dielectric resonator and a voltage-tuned phase shifter for the DRO's frequency tuning. The output frequency (9950 MHz to 10100 MHz, spaced by 10 MHz) which is the sum of both the L-band synthesizer and the DRO frequencies, is filtered by the band-pass filter 320 and the output signal F.sub.o is amplified by the output amplifier 340. Similarly to FIG. 1, the frequencies of the 4 SAW oscillators and of the DRO are stabilized by phase locking each oscillator to a high stability reference oscillator 420 (e.g. a crystal or other high stability oscillator). As in FIG. 1, the design (including temperature control)of all the oscillators must be controlled to ensure that the frequency drift with temperature and aging effects of each oscillator do not exceed the voltage-controlled frequency tuning range of each oscillator.
Assuming that the 8.5 GHz dielectric resonator in oscillator 400 is made of Barium Tetratitanate, its unloaded Q (Q.sub.U) is approximately 5000 which results in 3 dB bandwidth of 3.4 MHz when critically coupled to the oscillator circuit. This provides a useful voltage-controlled frequency tuning range on the order of .+-.850 kHz. Since resonators of this type are available with numerous values of frequency/temperature relationships, an average value of 2 ppm/.degree. C. (or 17 kHz/.degree. C.) is arbitrarily selected for this example. For an operating temperature range of 100.degree. C. the DRO frequency drift is 1.7 MHz without frequency stabilization. Therefore, if a PLL is used for DRO frequency stabilization and a linear frequency versus temperature relationship is assumed over the operating temperature range, the temperature range of the DRO must be reduced to less than 50.degree. C. to ensure that the PLL does not lose lock. This 50.degree. C. range requires very simple temperature control.
However, the advantage of wide oscillator frequency tuning range provided by low Q resonators is offset by the disadvantage of relatively poor phase noise performance. The predicted phase noise of this DRO is approximately -115 dBc/Hz at 10 kHz offset from the carrier, caused primarily by the low Q.sub.U (5000) of the resonator. Since the predicted phase noise of the L-band direct analog synthesizer 100 is -152 dBc/Hz at 10 kHz offset from the carrier, the predicted cumulative phase noise of the microwave synthesizer is limited by the phase noise of the DRO to -115 dBc/Hz at 10 kHz offset from the carrier.
The phase noise of the microwave synthesizer topology in FIG. 2 can be reduced significantly by use of ultra low noise microwave oscillators incorporating very high Q microwave resonators and very low phase noise microwave amplifiers (amplifiers with phase noise of -160 dBc/Hz or lower at 10 kHz offset from the carrier). Examples of very high Q resonators include sapphire "whispering gallery" resonators.sup.(3), sapphire disc resonators in enclosures lined with high temperature superconducting (HTS) films.sup.(4), and Distributed Bragg Reflector resonators.sup.(5). The measured Q.sub.U of X-band whispering gallery resonators is 200,000 at room temperature (300 K) and 20,000,000 at 77 K (with cryogenic cooling). The Q.sub.U of an X-band sapphire disc resonator in an enclosure lined with HTS films is 300,000 at 77 K and the Q.sub.U of an X-band Distributed Bragg Reflector resonator is approximately 600,000 at 300 K. As an example, the predicted phase noise of a 10 GHz whispering gallery resonator oscillator at 10 kHz offset from the carrier can be as low as -142 dBc/Hz at 300 K and -159 dBc/Hz at 77 K. Close to the carrier phase noise of these oscillators can be reduced significantly with frequency-locked noise degeneration loops.sup.(6). However, these loops do not impact the problem solved by this invention.
Many very high Q resonators exhibit a large frequency dependence on temperature. For example, the reported frequency variation of a 10 GHz sapphire whispering gallery resonator is 70 ppm/K.sup.(7) or 700 kHz/K at 300 K and 11.5 ppm/K.sup.(8) or 115 kHz/K at 77 K. The frequencies of oscillators using very high Q resonators cannot be stabilized by phase-locking to stable reference oscillators because the high Q values of the resonators result in oscillator voltage-controlled frequency tuning ranges that are much smaller than the frequency drift of the respective resonators over temperature. For example, for an X-band whispering gallery resonator Q.sub.U values of 200,000 at 300 K and 20,000,000 at 77 K, the 3 dB bandwidths are 100 kHz and 1 kHz, respectively, when critically coupled to the oscillator circuit. Under the condition of critical coupling, the loaded Q (Q.sub.L) of the resonator is equal to Q.sub.U /2. Since the effective voltage-controlled frequency tuning range of an oscillator can be realized over approximately 50% of the resonator's 3 dB bandwidth, the tuning range of the oscillator is .+-.25 kHz at 300 K and .+-.250 Hz at 77 K. Consequently, to phase-lock a whispering gallery resonator oscillator to a stable reference, the temperature range of the resonator must be reduced with temperature control not to exceed 0.035K at 300 K and 0.002K at 77 K. Such stringent temperature control requirements are impractical in most frequency synthesis applications.
A need exists, therefore, for a more practical ultra low noise microwave oscillator frequency stabilization method to facilitate the use of these oscillators in ultra low noise microwave synthesizers.